In recent years, plastic films, sheets, injection-molded products, pipes, extrusion-molded products, and blow-molded products are increasingly used in various industrial fields. In particular, polyolefin resins (olefin polymers) are widely used because of low cost and light weight, a high level of moldability, stiffness, impact strength, transparency, chemical resistance, and recyclability, and other reasons. In general, polyolefin resins are subjected to molding while kept in a melted state. In many cases, however, olefin homopolymers have insufficient melt properties such as insufficient fluidity and elongational viscosity, have difficulty in maintaining sufficient moldability, or have an insufficient level of solid state properties such as transparency and stiffness.
Among polyethylene resins, linear low density polyethylene (L-LDPE) obtained by catalytic polymerization of ethylene and α-olefin is known as a high-strength resin. However, it is difficult to provide reliable moldability for L-LDPE alone, and L-LDPE has disadvantages such as low transparency and stiffness. As a measure to compensate for these disadvantages, high-pressure process low-density polyethylene (HPLD) with high moldability or an olefin polymer as a modifier with a different molecular weight or a different density has been blended to improve the melt properties or the solid state properties.
Unfortunately, the use of HPLD as a modifier can cause a problem such as a reduction in impact strength although it can improve moldability, and the use of an olefin polymer with a different molecular weight or density can cause a problem such as insufficient moldability or degradation of transparency due to a widened distribution of molecular weight or copolymer composition.
The current enforcement of the Law for the Promotion of Sorted Collection and Recycling of Containers and Packages and the current trend toward resource conservation require a reduction in the consumption of raw material resins. From this point of view, there is an increasing demand for a reduction in the thickness of molded products. This demand requires an improvement of impact strength and stiffness (elastic modulus).
Reduction of the density of ethylene polymers is a well-known method for improving impact strength. However, this method can also reduce the stiffness (or make the polymers soft) and thus is not preferred. Attempts for the thickness reduction include, for example, the use of a combination of two specific ethylene-α-olefin copolymers with different densities and the use of a three-component blend composition containing a specific HPLD for improving moldability and transparency (see Patent Literature 1).
These methods can produce polyethylene resin compositions with high transparency and a good balance between impact strength and stiffness as compared with those of traditional compositions. In these methods, however, a reduction in impact strength is inevitably associated with the HPLD blending, and the blending of three ethylene polymers is considered to be economically disadvantageous in terms of stable supply of constant-quality products at an industrial level as compared with transitional methods.
In recent years, it has been reported that polyolefin resin-modifying ethylene polymers for improving moldability and resin strength at the same time are developed by utilizing a polymerization designing technique with a metallocene catalyst that allows a long-chain branching structure to be formed in ethylene polymers. Examples of such a technique include a technique in which an ethylene polymer containing long-chain branches exhibiting specific elongational viscosity behavior is used as a modifier and blended with the target polyolefin resin (see Patent Literature 2), a technique in which a low-density ethylene-propylene copolymer having a long-chain branching structure defined by a specific polymer molecular structure index and a specific intrinsic viscosity ratio is used as a modifier to form a resin composition (see Patent Literature 3), and a technique in which long-chain branching polyethylene with a wide molecular weight distribution exhibiting high flow activation energy is used as a modifier (see Patent Literature 4). These techniques can prevent a significant reduction in the impact strength of polyolefin resins, which would otherwise be caused by traditional modification with HPLD, but are still not able to prevent a reduction in strength or transparency and still at an insufficient improvement level because of insufficient design of the long chain branching ethylene polymer.
Under these circumstances, there has been continued development of a modifying ethylene polymer that can solve the problems with conventional modifying ethylene polymers and provide high moldability, a good balance between impact strength and stiffness, and high transparency, and there have been continued studies on a metallocene polymerization catalyst that is useful for the development of an ethylene polymer with such properties and capable of controlling a long-chain branching structure (see Patent Literatures 5 to 8). Among them, a transition metal catalyst comprising a specific cyclopentadienyl compound, which has been recently found by the inventor et al., is proposed as a highly active catalyst for ethylene-α-olefin copolymers with preferred long-chain branching (Patent Literature 8).